Declare only the tables and columns pixa writes and reads (closes #310)

Removes output_content and request_cache, which nothing writes, and every
column that was written but never read: the host, path, query, content_type
and fetched_at of source_content, the content_type and created_at of
variant_content, and the status_code, error_message and fetched_at of
negative_cache.

StoreSource no longer takes the fetch result and LookupSource returns only
the source key. StoreVariant no longer takes a content type and
StoreNegative no longer takes a status code or message, so
extractStatusCode and the status constants, which only fed status_code,
are gone. Reconciliation adopts a variant file without opening it, as its
row no longer records a content type. Tests that set or checked a removed
column follow.

Model: opus-5-5

internal/imgcache/cache.go

@@ -297,20 +297,19 @@ func (c *Cache) LookupSource(
 
 	key := req.Source.Key()
 
-	var foundKey string
+	var rowExists bool
 
 	err := c.db.QueryRowContext(ctx, `
-		SELECT source_key FROM source_content WHERE source_key = ?
-	`, string(key)).Scan(&foundKey)
-
-	if errors.Is(err, sql.ErrNoRows) {
-		return "", nil
-	}
-
+		SELECT EXISTS(SELECT 1 FROM source_content WHERE source_key = ?)
+	`, string(key)).Scan(&rowExists)
 	if err != nil {
 		return "", fmt.Errorf("failed to lookup source: %w", err)
 	}
 
+	if !rowExists {
+		return "", nil
+	}
+
 	// Verify the content file exists
 	if !c.srcContent.Exists(key) {
 		return "", nil